This invention relates to a blood pump, and more particularly, to a combined axial flow pump and motor that is to be disposed in the bloodstream of a patient to pump or assist in the pumping of blood throughout the patient's circulatory system, either intracorporeally or extracorporeally.
It is desired that a motor/pump of this type have as small a size as possible consistent with the pumping requirements of the device. Suspension of the rotor with respect to the stator is a key to miniaturization. Where it is possible to minimize the structure by which the rotor is suspended with respect to the stator, it becomes possible to minimize the overall diameter of the motor and pump combination.
It is also desired that a motor/pump of this type be constructed which requires neither radial seals that can break down and leak nor radial bearings that require a continuous flow of blood compatible purge fluid. Where it is possible to use a method of suspension of the rotor that operates solely in blood, rotary seals can be avoided and the need to continuously supply purge fluid can be eliminated.
It has been an objective of the present invention to provide an axial flow blood pump with an improved rotor suspension system which minimizes the size of the motor and pump combination, enables the pump to be suspended without radial seals and eliminates the need to continuously supply purge fluid.
This invention contemplates an axial flow blood pump and motor combination which includes a cylindrical housing, or conduit, adapted to be inserted into a patient's bloodstream. The pump and motor combination includes a rotor which, during operation, is radially suspended in the conduit solely by hydrodynamic bearings formed by the pumped blood flowing through the conduit. In effect, the blood "floats" the rotor within the conduit.
In accordance with the present invention, an axial flow blood pump includes a cylindrical conduit to be disposed in the bloodstream of a patient. A pump stator is fixedly mounted coaxially within the conduit and includes inlet and outlet sections, each of which has a set of radially directed stator vanes. A pump/motor rotor resides within the conduit between the inlet and outlet stator sections of the pump stator. A motor stator, located either externally or internally of the conduit, is adapted to create a magnetic flux within the conduit. The rotor carries permanent magnets which interact with the applied flux to rotate the rotor. The rotor also carries impeller blades which, upon rotation of the rotor in the conduit, drive blood from the inlet, through the conduit, and past the outlet. During operation, the rotor is suspended by a hydrodynamic bearing formed by the pumped blood flowing through a radial gap between the inside surface of the conduit and the rotor.
During rotor rotation, the external surfaces, and for some embodiments also the internal surfaces, of the rotor are oriented so that their motion with respect to the conduit and the pump stator fixed therein produce a pressure distribution on the rotor that supports the rotor radially. The pressure distribution also creates a pumping action causing blood to flow through a gap between the rotor and the inside surface of the conduit. Blood flow through this gap is further enhanced by the pressure generated axially across the rotor through the action of the impellers. The axial blood flow past the rotor which is caused by the combined pumping action of the hydrodynamic bearing and the pressure generated by the impellers is hereinafter referred to as leakage flow. The radial gap between the rotor and conduit may be in the range of about 0.001" to about 0.010", and preferably is about 0.003". Since it is this radial gap with the blood flowing through that provides the suspension for the rotor, the radial clearance of the rotor is extremely small. Furthermore, because blood flows directly through the gap which forms this hydrodynamic bearing, neither radial bearings nor radial bearing seals need to be included, nor is purge fluid required for bearing lubrication.
This invention contemplates a number of various structural configurations for radially suspending a rotor within a cylindrical pump housing solely by one or more hydrodynamic bearings. Of these different configurations, several are more suitable for extracorporeal use, while some of the others are more suitable for long term intracorporeal use. Still others may be used advantageously in either mode of operation.
In a first embodiment of the invention, the rotor is cylindrical. Each of the inlet and outlet sections of the pump stator has a plurality of axially-extending vanes with radial inner edges pressed into the external surface of an elongated stator hub which passes through the center of the cylindrical rotor. Located axially between the inlet and outlet stator sections, the cylindrical rotor has an outer surface which cooperates with the conduit inside cylindrical surface. The rotor outer surface and the conduit inner surface create one location wherein the relative radial motion therebetween creates an outer, annular blood flow gap which serves as a hydrodynamic bearing for the rotor. The permanent magnets carried by the rotor are located radially inside of this external blood flow gap.
Intermediate stator vanes, axially spaced from both the inlet and outlet stator vanes, project radially outwardly from the stator hub. On the inlet and outlet sides of the intermediate stator vanes, the stator hub includes two axially spaced reduced diameter portions with annular grooved surfaces. The rotor has two corresponding axially spaced sets of impeller blades. The blades of each set extend radially inwardly from the inside surface of the rotor cylinder, and their radially internal ends terminate in an impeller support ring. The two axially spaced impeller support rings of the impeller blades cooperate with the corresponding axially spaced grooved surfaces of the stator hub to create two axially aligned portions of an internal blood flow gap between the rotor and the stator hub. Thus, with this construction, there is an annular blood flow gap between the rotor outer surface and the conduit inner surface which serves as an outer hydrodynamic bearing, and there are two axially spaced portions of another annular blood flow gap between the two impeller support rings and the stator hub. These portions of this latter gap also serve as additional, inner hydrodynamic bearings for supporting the rotor during rotation.
Each of the axially spaced cylindrical surfaces on the stator hub is in the form of an annular groove that is a shallow U-shape in longitudinal section. Similarly, at the conduit inner surface, the stator inlet and outlet sections cooperate to form a shallow U-shaped annular groove in longitudinal section. These shallow U-shaped grooves axially capture the rotor and maintain it centered radially within the conduit. The force of the impeller tends to drive the rotor axially towards the inlet as it rotates during its blood pumping function. The rearward axial force is frictionally resisted by the engagement of radial surfaces between the rotor and the shallow grooves on the stator hubs and on the stator inlet section. Alternatively, thrust-resisting magnets may be mounted in the stator inlet section to form a thrust-resisting system.
In this first embodiment, and also in a second embodiment, to be described later, a cylindrical rotor is utilized and the impeller blades extend radially inwardly from the pole pieces. This places the pole pieces relatively close to the motor stator, thereby minimizing the magnetic air gap therebetween. The dimension of the magnetic air gap affects the power output of the motor. With all other factors equal, as the magnetic air gap of a motor increases, the power of the motor decreases. Thus, in general, the output of a pump driven by the motor will also decrease.
As explained in the parent application, an axial flow blood pump used as an extracorporeal device requires about 6.0 liters per minute of blood flow at about 300 mg Hg. To some degree, these flow output requirements dictate the structural configuration of the rotor. More particularly, when used extracorporeally, it is best to optimize the power output of the pump by designing a motor and stator configuration which has a minimum magnetic air gap. For this reason, the impeller blades extend radially inwardly from a cylindrical magnet housing.
On the other hand, when used intracorporeally, the motor power output requirement for the pump is significantly lower. While the required blood flow rate remains at about 6.0 liters per minute, the required pressure is lower by about a third, at a value of about 100 mg Hg. With the lower power output required for intracorporeal use, design considerations such as uninterrupted long term use and potential thrombus formation take precedence over the prior, primary consideration of minimizing magnetic air gap.
More specifically, by increasing the magnetic air gap above the dimension utilized in the first two embodiments, the rotor may be made rod-shaped rather than cylindrical and located on a central axis through the pump housing. This shape enables the impeller blades to be located radially outside of the rotor permanent magnets. Compared to the first two embodiments in which blood flows in two widely separated annular paths, both outside and inside a cylindrical rotor, this configuration results in a blood flow path which is entirely outside the radial exterior of the rotor. The third, fourth, fifth, sixth and seventh embodiments utilize a rod-shaped, radially centered rotor to provide an entirely external blood flow path.
Generally, by utilizing a blood flow path around the outside of the rotor, the blood flows in more of a direct line from the inlet to the outlet. Proportionately, radially directed blood flow is reduced, and axially directed blood flow is increased. As a result, the blood flow path is generally straighter, with a reduced possibility that thrombus, or blood clotting, will occur within narrow passages or sharp turns within the structural components of the pump. Generally, such narrow passages pose the greatest threat of blood cell aggregation.
As seen by variations among the third, fourth, fifth, sixth and seventh embodiments, a number of structural modifications of the rotor, the stator and the conduit are possible. The structural differences among these particular embodiments address different concerns related to long term intracorporeal use. However, the most notable of these concerns is that of maximizing the proportion of axially directed blood flow with respect to radially directed blood flow. This enhances the continuous washing of blood contacting surfaces and minimizes stagnation of the blood, thereby reducing the possibility of thrombus.
Additional modifications described in the eighth and ninth embodiments involve the use of a cylindrical rotor, but with the motor stator located inside the conduit and integral with the pump stator, rather than external to the conduit and removed from the pump stator.
For both cylindrical and the rod-shaped rotor embodiments, the invention contemplates the use of a single stage of impeller blades, as opposed to two or even three sets of axially displaced impeller blades. Embodiments ten and eleven relate to an axial flow blood pump with a single stage of impeller blades.
For all embodiments of the axial flow blood pump of this invention, the rotor is radially supported hydrodynamically, solely by the pumped blood. In effect, the blood floats the rotor. In operation, the shear stress and washing action should be high enough to provide sufficient force to prohibit blood cells from aggregating and thereby causing thrombus. However, it is critically important to assure that shear stress is not so high as to cause blood cell destruction.
Furthermore, cell destruction is due not only to shear stress, but also to exposure time of the cells to shear stress. Thus, it is important to provide a rate of leakage flow through the pump which is high enough to minimize residence time of the blood cells in the various gaps. A level of shear stress and exposure time at approximately the threshold level for cell destruction will satisfy both requirements of minimizing thrombus formation and minimizing blood cell destruction. The parameters that contribute to shear stress are the sizes of the gaps which form the flow path, and the velocity of the blood cells through these gaps. Residence time in the gap flow path is dependent upon the overall rate of leakage flow, which in turn is dependent upon the size of each of the gaps. The larger the gap size, the lower is the shear stress and the higher is the rate of leakage flow. Additionally, the greater the proportion of axially directed blood flow with respect to radially directed blood flow, the lesser will be the residence time of the blood cells in the gap. However, if the gaps are too large, the rotor will wobble during rotation. If the gaps are too small, the dwell time will be excessive, and too much viscous friction of the blood cells will be created.
With respect to the first embodiment, in order to minimize cell damage in the gaps, shear stress should be maintained below 2500 dynes per square centimeter and the residence time of blood cells in the outer gap should be below 0.1 second. However, the outer annular gap size must be kept low to minimize violent eccentric motion of the rotor with respect to the stator.
Again, with respect to the first two embodiments, blood cell destruction within the main flow path of the pump, i.e., inside the cylindrical rotor and through the impeller and stator vanes, is also dependent upon shear stress. Since the velocity of a blood cell with respect to the pump stator surfaces affects shear stress, it is sufficient to minimize velocity through the main flow path as a means of minimizing cell destruction. The velocity of blood pumped through the conduit should be below 1000 centimeters per second and preferably below 500 centimeters per second.
It is believed that these velocity parameters may also be applied to the other embodiments of the invention to achieve preferable velocity of blood flow. For each embodiment, the rotational speed of the rotor may be controlled so as to provide the desired velocity of blood flow. For embodiments one, two and ten, to minimize the chance of hemolysis to the blood, it is believed that rotor speed should be in the range of 10,000 to about 13,000 rpm, and preferably as close to about 10,000 rpm as possible. For the third, fourth, fifth, sixth and seventh embodiments, rotor speed should preferably be about 8000 rpm. For the eighth and ninth embodiments, rotor speed should be about 6,000 rpm.
If desired, the rotor may be rotatably driven so as to produce either continuous flow of blood or pulsatile flow of blood. Pulsatile flow may be achieved by cycling the rotational speed of the rotor between a fast and a slow speed at a frequency that corresponds to a human pulse rate.
The features and objectives of the invention will become more readily apparent from the following detailed description and the accompanying drawings.